But just over a year ago, a mass immunization campaign in West Africa was able to deliver over 150,000 doses without using ice, Nature News reports. The vaccines remained viable, with no losses from excess heat -- a real game changer, experts say.

Almost all vaccines are sensitive to damage from excessive heat or cold, and they’re licensed by regulatory groups exclusively for use under conditions where they’re stored and distributed between 2 and 8 degrees Celsius, Nature explains:

But many vaccines are actually thermostable outside that range, and some newer ones in particular can often remain viable for days, weeks or even months after exposure to higher temperatures. This has prompted interest in a 'controlled temperature chain' (CTC) that would allow vaccines to be stored for short periods at higher temperatures for the 'last mile' of distribution, something that would be especially helpful in hot developing countries where electricity and refrigeration is lacking.

The anti-meningitis campaign carried out in a rural district in Benin in December 2012 kept the vaccine stored at temperatures of up to 40 degrees Celsius for up to four days. The details of the trial, Nature reports:

Health workers delivered MenAfriVac -- the first vaccine to get licensing approval for CTC distribution -- to more than 155,000 people in 150 villages using coolers with no icepacks (pictured).

All the vials had stickers that measured the cumulative heat exposure over time and would change color if a vaccine needed to be discarded.

Additionally, each cooler had a heat-sensitive card that changed color if the peak temperature exceeded 40 degrees Celsius.

Each of the 15,000 vials contained 10 doses. None of the vials had to be discarded for heat damage. Nine vials had to be thrown away because they were stored at room temperature for longer than the approved four days -- not because the stickers or cards indicated that they’d exceeded their heat limits.

The researchers hope their findings will open the way to other vaccines being approved for CTCs. "For a long time, there was a feeling that this couldn't be done," study coauthor Simona Zipursky of the World Health Organization tells Nature. "Manufacturers wouldn't do CTC studies, regulators wouldn't approve CTC licenses, the WHO wouldn't develop CTC guidance and countries wouldn't want to deviate from the dogma of 2–8 °C."

Not only would approving CTC protocol ease the strain on overstretched supply systems, Nature explains, but it would also provide important savings: Delivering MenAfriVac using a CTC would cut costs in half.